Much research in the area of biopharmaceutics is directed toward the development of effective implantable drug delivery vehicles. Such vehicles must be biocompatible and also must be capable of protecting the activity of the biologically active agent they are intended to deliver. Many biologically active agents are labile and easily lose activity when they are incorporated into a delivery material. Preservation of protein activity has posed particularly difficult problems.
Calcium phosphate ceramics have been studied as potential drug delivery systems due to their well known biocompatibility and their affinity for protein reagents (see, for example, IJntema et al., Int. J. Pharm. 112:215, 1994; Itokazu et al., J. Orth. Surg. 2:47, 1994; Shinto et al., J. Bone Joint Surg. 74-B:600, 1992; Uchida et al., J. Orth. Res. 10:440, 1992). However, the reactions employed to produce known calcium phosphate ceramic materials typically require elevated temperatures and/or pressures, and also require the presence of acids or bases. Because most biologically active agents would be destroyed by one or more of the conditions required to produce the ceramic, the biologically active agents can only be loaded in after the material is produced, which can limit the amount and type of agent that can be delivered.
Also, although a number of calcium phosphate materials have been referred to as xe2x80x9cresorbablexe2x80x9d, such compounds, usually comprising or derived from tricalcium phosphate, tetracalcium phosphate or hydroxyapatite are in fact only weakly resorbable. Of the group, the tricalcium phosphate compounds have been demonstrated to be the most resorbable and, after many years of study, they are still not widely used in clinical settings. The tricalcium phosphates are known to have lengthy and somewhat unpredictable resorption profiles, generally requiring in excess of one year for resorption. Unless steps are taken to produce extremely porous or channeled tricalcium phosphates, these compounds are not replaced by bone. Recent studies have lead to the conclusion that the xe2x80x9cbiodegradation of TCP, which is higher than that of [hydroxyapatite], is not sufficientxe2x80x9d (Berger et al., Biomaterials, 16:1241, 1995).
Tetracalcium phosphate and hydroxyapatite derived compounds are also only weakly resorbable. Published reports of tetracalcium phosphate fillers generally describe partial resorption over long periods of time. For example, as reported by Horioglu et al., it is not uncommon for such materials to require 30 months for 80% resorption (Soc. for Bionatenals, pg. 198, Mar. 18-22, 1995). Also, many reports that describe xe2x80x9cresorptionxe2x80x9d of calcium phosphate materials do not actually demonstrate resorption because the authors do not rule out, for example, migration of the vehicle from the implant site (see, for example, IJntema et al., supra).
There remains a need for the development of a drug delivery vehicle that is biocompatible, fully resorbable, and not detrimental to drug activity. Preferably, the resorption rate of the material should be modifiable in known ways. Also, the material should be easy to manufacture, preferably using mild reaction conditions.
xe2x80x9cAmorphousxe2x80x9dxe2x80x94By xe2x80x9camorphousxe2x80x9d as that term is used here, it is meant a material with significant amorphous character. Significant amorphous character contemplates greater than 75% amorphous content, preferably greater than 90% amorphous content, and is characterized by a broad, featureless X-ray diffraction pattern. It is recognized that a small degree of crystallinity may exist in the material. However, for the amorphous precursor materials of the present invention, it is preferable that the degree of crystallinity be less than that desired in the product material.
xe2x80x9cBioactivexe2x80x9dxe2x80x94xe2x80x9cBioactivexe2x80x9d refers to a material that induces hard tissue formation in and about the implant. When implanted in soft tissue, the bioactivity may also require the presence of a growth or trophic factor, or the seeding of the implant with a hard tissue forming cell type.
xe2x80x9cBiocompatible xe2x80x9dxe2x80x94The term xe2x80x9cbiocompatiblexe2x80x9d, as used herein, means that the material does not elicit a substantial detrimental response in the host. There is always concern, when a foreign object is introduced into a living body, that the object will induce an immune reaction, such as an inflammatory response that will have negative effects on the host. For example, although hydroxyapatite is generally considered to be xe2x80x9cbiocompatiblexe2x80x9d, significant inflammation and tissue necrosis have been observed when crystalline hydroxyapatite microcarriers are inserted intramuscularly in animals (see, for example, IJntema et al., Int. J. Pharm 112:215 (1994)).
xe2x80x9cBioresorbablexe2x80x9dxe2x80x94xe2x80x9cBioresorbablexe2x80x9d refers to the ability of a material to be resorbed in vivo. xe2x80x9cFullxe2x80x9d resorption means that no significant extracellular fragments remain. The resorption process involves elimination of the original implant materials through the action of body fluids, enzymes or cells. Resorbed calcium phosphate may, for example, be redeposited as bone mineral, or by being otherwise reutilized within the body, or excreted. xe2x80x9cStrongly bioresorbablexe2x80x9d, as that term is used herein, means that at least 80% of the total mass of material implanted intramuscularly or subcutaneously is resorbed within one year. In preferred embodiments of the invention, the strongly resorbing PCA calcium phosphate is characterized in that, when at least 1 g (preferably 1-5 g) of PCA material is implanted at a subcutaneous or intramuscular site, at least 80% of the material is resorbed within one year. In more preferred embodiments, the material will be resorbed within nine months, six months, three months, and ideally one month. Furthermore, particularly preferred materials are characterized in that they can be fully resorbed in the stated time periods. For the purpose of this disclosure, xe2x80x9cweaklyxe2x80x9d resorbable means that less than 80% of the starting material is resorbed after one year.
xe2x80x9cEffective Amountxe2x80x9dxe2x80x94An effective amount of a biologically active agent is an amount sufficient to elicit a desired biological response.
xe2x80x9cHardeningxe2x80x9dxe2x80x94xe2x80x9cHardeningxe2x80x9d refers to the process by which the hydrated precursor is transformed into a hardened PCA material. The PCA material is considered to be xe2x80x9chardenedxe2x80x9d when it is a substantially non-formable solid. Such a hardened PCA material has minimal compressibility and tends to undergo plastic as opposed to elastic deformation.
xe2x80x9cHydrated precursorxe2x80x9dxe2x80x94The term xe2x80x9chydrated precursorxe2x80x9d, as used herein, refers to the paste or putty formed by hydration of the dry PCA precursors in the presence of a limited amount of aqueous solution (i.e., less than approximately 1 mL aqueous solution/1 g precursor powder). The hydrated precursor may comprise both reactants and products, in various combinations, depending on the extent to which the conversion has progressed. Both the xe2x80x9cinjectablexe2x80x9d and xe2x80x9cformablexe2x80x9d PCA precursor pastes described herein are hydrated precursors. Preferred xe2x80x9cinjectablexe2x80x9d hydrated precursors have a consistency appropriate for delivery through an 18 gauge needle.
xe2x80x9cPoorly crystalline apatitic calcium phosphatexe2x80x9d, xe2x80x9cPCA calcium phosphatexe2x80x9d and xe2x80x9cPCA materialxe2x80x9d, as those terms are used herein, describe a synthetic poorly crystalline apatitic calcium phosphate. The PCA material is not necessarily restricted to a single calcium phosphate phase provided it has the characteristic XRD and FTIR pattern. A PCA calcium phosphate has substantially the same X-ray diffraction spectrum as bone. The spectrum is generally characterized by only two broad peaks in the region of 20-35xc2x0 with one centered at 26xc2x0 and the other centered at 32xc2x0. An additional broad shoulder occurs at approximately 2xcex8=29 and another may be present at approximately 2xcex8=33.6. Absent from the spectra are any additional sharp peaks or sharp shoulders characteristic of crystalline hydroxyapatite occurring in the range of 2xcex8=27-34. In particular, there are no sharp peaks or shoulders corresponding to Miller""s Indices for 210, 112, or 300 for hydroxyapatite. It is further characterized by FTIR peaks at 563 cmxe2x88x921, 1034 cmxe2x88x921, 1638 cmxe2x88x921 and 3432 cmxe2x88x921 (xc2x12 cmxe2x88x921). Sharp shoulders are observed at 603 cmxe2x88x921 and 875 cmxe2x88x921, with a doublet having maxima at 1422 cmxe2x88x921 and 1457 cmxe2x88x921.
xe2x80x9cPromoterxe2x80x9d xe2x80x94The term xe2x80x9cpromoterxe2x80x9d, as used herein, describes a material or treatment that promotes hardening of a hydrated precursor and may enhance the ACP to PCA calcium phosphate conversion. Some promoters participate in the conversion and are incorporated into the product PCA material; others, known as xe2x80x9cpassivexe2x80x9d promoters, do not participate.
xe2x80x9cReactivexe2x80x9dxe2x80x94xe2x80x9cReactivexe2x80x9d is used herein to refer to the ability of an amorphous calcium phosphate when mixed with liquid to form a hydrated precursor to undergo conversion to the PCA material of the present invention in the presence of a promoter in association with hardening of the precursor materials. Preferred ACPs are characterized by an ability to convert completely, an ability to convert quickly with hardening, an ability to undergo conversion with otherwise inert compounds and/or an ability to convert into a substantially homogeneous PCA material. Where the ACP is reacted with a second calcium phosphate, the xe2x80x9cconversionxe2x80x9d can encompass conversion of both the ACP and the second calcium phosphate. The degree of hardening and the kinetics of the hardening process are also important elements of reactivity. Some ACPs are more reactive than others. An ACP is considered xe2x80x9chighly reactivexe2x80x9d if it undergoes conversion and hardening to a PCA material in the presence of a weak promoter, such as dicalcium phosphate dihydrate (xe2x80x9cDCPDxe2x80x9d) with a grain distribution containing a significant fraction of grain sizes greater than 100 xcexcm. Preferred highly reactive ACPs produce a hardened PCA material in the presence of weakly promoting DCPD and water at 37xc2x0 C. in less than twelve hours, with hardening being substantially complete in about one to five hours, and ideally 10-30 minutes.
The present invention provides a delivery vehicle for biologically active agents that has excellent biocompatibility and is bioresorbable. The delivery vehicle is comprised of a synthetic, poorly crystalline apatitic (PCA) calcium phosphate material. The PCA material of the present invention is strongly bioresorbable. That is, when an implant comprising at least 1 g of material is implanted in pellet form in an intramuscular or subcutaneous site, at least approximately 80% of the material is resorbed within one year, preferably within 9 months, 6 months, 3 months, and, ideally 1 month. More preferably, at least 80% of a 5 g implant is resorbed within these time frames. It will be appreciated that the conformation of the material (e.g., in a sphere as compared with a rod or other shape) may affect is resorption rate. Furthermore, the resorption rate of the delivery vehicle can be varied through its manner of preparation.
The synthetic PCA material utilized in the delivery vehicle of the present invention is compatible with a wide array of biologically active agents and can be employed to deliver agents to any of a variety of sites in the body. The material is characterized by a distinctive X-ray diffraction pattern that reveals its poor crystallinity. Preferably, the material has a calcium to phosphate ratio in the range of about 1.1 to 1.9. More preferably, this ratio is in the range of about 1.3 to 1.5.
In preferred embodiments of the present invention, the synthetic PCA material is formed in a reaction in which at least one amorphous calcium phosphate (ACP) precursor is exposed to a promoter. In particularly preferred embodiments, the promoter comprises a second calcium phosphate material. The reaction conditions employed to produce the PCA material utilized in the present invention are mild, so that biological agents can be incorporated into the material during the formation reaction, if desired. Alternatively, the agents may be incorporated after the delivery vehicle is made. The delivery vehicle material may be formed into any of a variety of useful delivery shapes, either before or after the introduction of biologically active agent, and may be delivered to the site by, for example, injection or surgical implantation. The material may be introduced into a site in a wet, non-hardened state (i.e., as a hydrated precursor) and allowed to harden in situ. The vehicle may alternately be hardened in wiro at an elevated temperature, generally at or above 37xc2x0 C., and thereafter surgically implanted into a subject (animal or human). The device may be fabricated in vitro either in the presence or absence of the therapeutic agent. The therapeutic agent may be added post-hardening by exposing the pre-formed vehicle to the agent. One advantage of the delivery system of the present invention is that it allows a high local concentration of drug to be achieved, which is particularly useful with drugs that have toxic side effects and also with labile drugs.
The present invention therefore provides vehicles for delivering biologically active agents, which vehicles comprise a PCA calcium phosphate and a biologically active agent. The inventive vehicles optionally comprise, for example, other bioresorbable materials, erosion rate modifiers, cells, or other factors that modify one or more characteristics of the vehicle (such as its strength, adherence, injectability, frictional characteristics, etc.).
The invention also provides methods of preparing delivery vehicles, of altering delivery vehicle characteristics, and of delivering biologically active agents to a site. Preferred delivery sites include both in vitro and in vivo sites. The delivery vehicles of the invention are suitable for delivery into human or animal sites. Preferred in vivo sites include bony sites, intramuscular sites, interperitoneal sites, subcutaneous sites, central nervous system sites, and occular sites.